I am replacing my exterior door with a new framed, exterior door (36") that has rough opening of 37.5". The wall-to-wall gap is 40". So, I have a total gap of 2.5" (= 40" - 37.5"). I.e. a gap of 1.25" on each side between the door-jamb and the brick wall

My Thoughts:

Use TWO normal 4 9/16 jamb frame lumber of 1" thickness. This would reduce the TOTAL gap to 1/2" (= 40" - 2 x 1" - 37.5"); i.e., 1/4" gap on each side of the door?


  1. Is the gap of 2.5" (1.25" on each side) too much ? If so, what is the normal/acceptable gap?
  2. What is the best way to fill such a gap if a gap of 2.5" (1.25" on each side) too much?
  3. What material/lumber should I use to reduce this gap to a normal gap of 3/4" (in US Midwest/Chicago)
  4. Is a gap of 1/4" on each side of the door between the jamb and brick-wall okay of is it too small (for Chicago conditions, late-spring installation)?
  5. What is the minimum and maximum gap for Chicago conditions?


1 Answer 1


First off a rough opening that is larger than you need is way better than an opening that is narrower than you need. :)

Yes, 2 1/2" difference from net outside of jamb width to opening is excessive. And yet, it does not seem too big to be covered with a reasonably sized door trim (casing) after install.

Typically a rough opening for a door is 2" wider than the actual door size. This allows for the 2- 3/4" jambs (1 1/2" total) and leaves 1/2" of shim room for setting the door plumb and true. Exterior doors can sometimes be quite heavy and sometimes the hinge screws will protrude through the back of the jamb- in these cases I make my rough opening 3/8"-1/2" wider than usual to make setting of the door jambs easier. The other factor in your opening size is how plumb the jamb is. If it is out too much your opening size will need to be wider so you have room to shim the jamb plumb.

Both sides of your door jamb need to be well anchored to the wall- the hinge side because it carries the weight of the door and the latch side because, intentionally or not, doors can slam shut very hard. So whatever you need to add to make your door opening the size you want - be sure that it is securely fastened to your wall opening. As far as what material this should be, here we use wood treated with a preservative against bugs and moisture- I am not sure that is necessary where you are. It can be a full height board or it can be blocks attached at your shim points. Just make sure to properly weather seal whatever you do with either insulation tucked into any gaps or spray foam to fill any gaps. (It gets cold there right ?). I actually think your idea of an extra door jamb on each side of the opening is fine but I am not sure that will leave you enough shim room. Is it possible you can use a 2x material (1 1/2" net) on the hinge side only ? This way you could get good secure fastening to your brick wall and have good fastening for the hinge side of your jamb ? This will not work if you are trying to keep the door centered left to right.

  • Kyle: Thank you SO very much! I will go with your recommendation -- have 3/4" AC2 above-ground treated 4 9/6" boards on each side of the door; and as you pointed out, I'll hence have 1/2" gap on each side. I plan on screwing these AC2 with at least four 2 1/2" screws on each side so that the frame is solid, stable and well anchored to the brick wall; I will use two 2 1/2" screws on each hinge; this will make sure that the 6 hinge screws all the way. This should secure the hinge-side pretty well (to address the hinge-stability point you made).
    – user97485
    Apr 26, 2022 at 3:15

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